Royal Army Medical Corps training pamphlet / War Office.

Date:
1944
Catalogue details

Licence: Public Domain Mark

Credit: Royal Army Medical Corps training pamphlet / War Office. Source: Wellcome Collection.

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    the nerve-endings of the optic nerve, and thus produces impulses which the nerve conveys to the brain. By analysing the impulses received through the optic nerve, the brain can appreciate the shape, colour, size and position of objects within the range of vision. The fact that each eye sees the object from a slightly different angle gives stereoscopic vision, which makes judgment of distance much easier. 86. Defective vision.—In short sight the eye is abnormally long from front to back. Consequently the image of distant objects, instead of being focussed on the retina, is focussed a little in front of it, and being out of focus is blurred. In long sight the eye is abnormally short and the image of near objects is focussed behind the retina, making them blurred. These and other errors are corrected by means of spectacles which focus the image on the retina. In later life spectacles are almost always required for near vision, because, as age advances, the lens becomes less elastic and accommodation is therefore more difficult. CHAPTER 12 THE EAR 87. For purposes of description, the organ of hearing is divided into three parts—the external ear, the middle ear and the internal ear. The function of the external ear and middle ear is to transmit sound waves to the internal ear, which analyses them and reports its findings to the brain. 88. The external ear.—The visible projecting ear is merely an external flap, made of cartilage covered with skin, which guards the ear-hole. From this hole the external auditory canal leads to the ear-drum. The “ external ’’ ear includes not only the external flap, but also the external auditory canal—+.e. the whole ear as far as the drum (Fig. 32). The canal, from ear-hole to drum, is about 1} inches long, and is slightly curved. The skin lining its outer half is hairy and contains wax-producing glands; the hairs and wax hinder insects and other small ‘‘ foreign bodies ”’ from reaching the delicate drum. The inner half is a bony tunnel lined with thinner skin. It ends blindly at the ear-drum (Fig. 32), a thin parchment-like membrane which vibrates with every » sound.